1. Field of the Invention
The present invention relates to: an optical multi-layer thin film including a layer made of a high-refractive-index material and an outermost layer made of a low-refractive-index material; an optical element in which the optical multi-layer thin film is stacked, and which is used for an optical instrument such as a camera, a microscope, binoculars, and a projector; and a producing method for forming the optical multi-layer thin film.
2. Related Background Art
The surface of each lens, such as a camera lens and an objective lens, constituting an optical system is coated with an anti-reflection film to reduce reflection from the lens. Generally, an optical thin film is formed by a dry method (dry process). As the dry process, a vacuum deposition method, a sputtering method, a CVD (Chemical Vapor Deposition) method, or the like, is used. It is known that, in order to obtain a high-performance anti-reflection film having a low reflectance over a wide wavelength band or a wide angular band, a multi-layer film should be formed by combining multiple coating materials having different refractive indexes from each other.
When a multi-layer anti-reflection film is designed and formed by the vacuum deposition method, deposition substances of various refractive indexes are selected for use. Generally, TiO2 having a refractive index of 2.4 to 2.7 is utilized as the highest-refractive-index substance, while MgF2 having a refractive index of 1.38 is utilized as the lowest-refractive-index substance. Typical deposition substances with refractive indexes between the highest and the lowest are Al2O3 of 1.65, ZrO2 of 2.0, and Ta2O5 of 2.16. A deposition substance having a refractive index of 1.65 to 2.0 as well as practical applicability is yet to be found.
For example, Japanese Unexamined Patent Application Publication No. 2000-171609 (Document 1) proposes that an intermediate refractive index film of from 1.65 to 2.0 be obtained by use of a mixture. It is stated, here, that an intermediate refractive index film in a range of 1.6 to 1.9 is obtained by use of a deposition substance made of samarium oxide and aluminum oxide.
However, it has been considerably difficult to obtain a refractive index of between 1.65 and 2.0 by use of a mixture. This is because, in order to obtain a certain refractive index by continuously changing refractive indexes, modification has to be made to the composition itself of the deposition substance. Furthermore, even when the mixture is used, some range of refractive index is still difficult to obtain.
Moreover, for example, Japanese Unexamined Patent Application Publication No. Hei 8-122501 (Document 2) and the like propose that an anti-reflection film be formed by a wet film-formation method. In this wet film-formation method, the film is formed by applying a liquid onto a substrate by a spin coating method, a dipping method, a spray method, a roll coating method, or the like, and by drying and heating the liquid-applied substrate. As an advantage of the wet film-formation method, a large-sized apparatus is unnecessary unlike the dry process. Moreover, the film can be formed under the atmosphere, so that the cost can be reduced significantly.
However, in the wet film-formation method, it is difficult to adjust the refractive index and the film thickness accurately in comparison with a vacuum deposition method and the like. When an excellent anti-reflection property is to be obtained, a larger number of layers have to be stacked. Meanwhile, the application and drying steps need to be performed for each layer. For these reasons, the formation of multi-layer film requires a lot of works. When a multi-layer film is formed by the wet film-formation method, the number of layers that can be stacked is approximately three layers at most in practice.
When the excellent property is obtained with the small total layer number such as two or three layers, at least the uppermost layer must be a low refractive index film having a refractive index of 1.30 or lower. It has been found from a simulation result that, by making the uppermost layer be the low refractive index film having a refractive index of 1.30 or lower, an optical performance can be improved significantly. Specifically, the significantly improved effects can be obtained: a wider wavelength band in which the reflectance can be suppressed to be low; and a larger incident angle at which the reflectance of incident light over a wide angular range in addition to normal incident light can be suppressed to be low.
It is known that such a low refractive index film can be obtained by forming a porous film. Generally, the wet method is known to be suitable for obtaining such a porous film.
For example, International Publication No. WO02/18982 (Document 3) discloses a method for forming a porous MgF2 film. In this disclosed method, a MgF2 sol solution is prepared and heat-treated at a high temperature and high pressure using an autoclave. Accordingly, the MgF2 fine particles are grown, crystallized, and deposited to form a film. Even at this point, pores between the fine particles do not collapse, and thus a high porosity can be maintained. As a result, the film becomes porous, and the refractive index can be reduced significantly in comparison with a compacted film.